1. Field of the Invention
The present invention relates to a recovery of a terminal-station apparatus connecting a terminal apparatus to an exchange from an abnormality.
2. Description of the Related Art
A terminal such as a PC is connected to a terminal apparatus. A plurality of terminal apparatuses is connected to a terminal-station apparatus through a coupler. The terminal-station apparatus is connected to a plurality of exchanges. Communications between a terminal apparatus and the terminal-station apparatus in the going-up and going-down directions are carried out through a channel allocated by the terminal-station apparatus to the terminal apparatus. An apparatus number is assigned to each of the terminal apparatuses. Channels are allocated to the terminal apparatuses in ascending order of apparatus numbers. One channel or a plurality of channels are allocated to a terminal apparatus in dependence on the width of a band required by the terminal apparatus. If necessary, the width of the band can also be increased dynamically. When receiving data through a channel allocated to a terminal apparatus, the terminal-station apparatus maps the channel onto a channel of the exchange serving as the destination of the data and transmits the data to the exchange through the channel allocated to the exchange. When receiving data from an exchange through a channel allocated to the exchange, on the other hand, the terminal-station apparatus maps the channel onto a channel allocated to the terminal station serving as the destination of the data and transmits the data to the terminal station through the channel allocated to the terminal station.
FIG. 18 is a diagram showing the configuration of the conventional transmission network. As shown in FIG. 18, the transmission network includes a plurality of terminal apparatuses 2#ij where i=1 to m and j=1 to n, a coupler 4#i, a terminal-station apparatus 6 and a plurality of exchanges 8#k where k=1 to p. A terminal apparatus 2#ij is connected to the terminal-station apparatus 6 by typically an optical fiber. By the same token, the terminal-station apparatus 6 is connected to an exchange 8#k by typically an optical fiber. The terminal-station apparatus 6 includes a plurality of subscriber-line termination units 10#i where i=1 to m, a cross-connect unit 12 and a plurality of exchange connection units 14#k where k=1 to p.
FIG. 19 is a diagram showing details of components for going-up and going-down channels in the terminal-station apparatus 6. As shown in the figure, for the going-up channels, a subscriber-line termination unit 10#i includes a plurality of going-up reception channel units 20#iq where q=1, 2 and so on, a switch 22#i and a plurality of going-up transmission channel units 24#iq where q=1, 2 and so on. For the going-down channels, on the other hand, the subscriber-line termination unit 10#i includes a plurality of going-down reception channel units 44#iq where q=1, 2 and so on, a the switch 42#i and a plurality of going-down transmission channel units 40#iq where q=1, 2 and so on. By the same token, for the going-up channels, the cross-connection unit 12 includes a plurality of going-up reception channel units 30#iq where i=1 to m and q=1, 2 and so on, a switch 32 and a plurality of going-up transmission channel units 34#iq where i=1 to m and q=1, 2 and so on. For the going-down channels, on the other hand, the cross-connection unit 12 includes a plurality of going-down reception channel units 54#iq where i=1 to m and q=1, 2 and so on, a switch 52 and a plurality of going-down transmission channel units 50#iq where i=1 to m and q=1, 2 and so on. The going-up transmission channel units 24#iq are connected to the going-up reception channel units 30#iq respectively. In the same way, the going-down reception channel units 44#iq are connected to the going-down transmission channel units 50#iq respectively.
A signal of a going-up channel is explained by referring to FIGS. 18 and 19. A signal of a going-up channel is supplied from a terminal apparatus 2#ij and output to an exchange 8#p. In a subscriber-line termination unit 10#i for accommodating terminal apparatuses 2#ij, the going-up reception channel units 20# are assigned to the terminal apparatuses 2#ij in ascending order of apparatus numbers assigned to the terminal apparatuses 2#ij. To put it in detail, a going-up channel is newly assigned to a terminal apparatus 2#ij having an apparatus number higher than the apparatus number of a terminal apparatus 2#ij already having a going-up channel assigned thereto, and cataloged going-up reception channel numbers are updated. In addition, the going-up reception channel is also assigned to the corresponding going-up transmission channel unit 24#cd. In the subscriber-line termination unit 10#i, going-up channels are assigned to going-up transmission channel units 24#ef connected to going-up reception channel units 20#ab in an order the channels are cataloged. In this case, no already assigned transmission channel numbers are updated. Consider a case in which the terminal apparatuses 2#10 and 2#12 having apparatus numbers of 0 and 2 respectively have already been connected. When the terminal apparatus 2#11 is newly connected, no already assigned transmission channel numbers are updated in order to avoid effects on terminal apparatuses already in communication. The going-up transmission channel unit 24#ef is connected to the going-up reception channel unit 30#ef employed in the cross-connect unit 12. Thus, in the cross-connect unit 12, the going-up reception channel unit 30#ef needs to be connected to the going-up transmission channel unit 34#ab.
A communication abnormality may occur in the terminal-station apparatus between a terminal apparatus and an exchange. Communication abnormalities caused by the terminal-station apparatus are listed as follows:    a: A subscriber-line termination unit is not capable of receiving data correctly due to a problem of the reception channel unit employed in the subscriber-line termination unit.    b: Data cannot be propagated correctly due to an abnormality occurring in the switch connecting the reception channel unit and the transmission channel unit in a subscriber-line termination unit.    c: The cross-connect unit is not capable of receiving data correctly due to a problem of the transmission channel unit employed in a subscriber-line termination unit.    d: The cross-connect unit is not capable of receiving data correctly due to a problem of the reception channel unit employed in the cross-connect unit.    e: The cross-connect unit is not capable of receiving data correctly due to a problem of the switch connecting the reception channel unit to the transmission channel unit in the cross-connect unit.    f: An exchange connection unit is not capable of receiving data correctly due to a problem of the transmission channel unit employed in the cross-connect unit.    g: An exchange connection unit is not capable of receiving data correctly due to a problem of a reception channel unit employed in the exchange connection unit.    h: An exchange is not capable of receiving data correctly due to a problem of the transmission channel unit employed in the exchange connection unit.
The failures described above can be handled by adoption of the following conventional methods:    1: Each unit employed in the terminal-station apparatus is designed into a redundant configuration. As described above, units employed in the terminal-station apparatus are the subscriber-line termination unit, the cross-connect unit and the exchange connection unit. With such a redundant configuration, when a working system in the configuration fails, a spare system in the configuration is used as a substitute for the working system. This method is referred to as related art 1.    2: The connection of a terminal apparatus and an exchange is changed by reconnecting the terminal to another exchange. This method is referred to as related art 2.
However, the above conventional methods have the following problems. In related art 1, if the cost does not permit a redundant configuration, there is raised a problem that a failure causes an immediate communication service suspension. With related art 2, on the other hand, a route connecting a terminal apparatus and an exchange is changed. Thus, setting on the exchange side needs to be changed too.